It has long been the goal to be able to provide a deployable miniaturized wide band antenna which can accommodate a number of frequency bands and more particularly to be able to provide a reduced size antenna suitable for being deployed in the field whereby the antenna can be packaged in a small volume and then deployed when required.
Such antennas have application in multiple military environments in which it is desired to have a mobile base station that can be easily transported from one place to another, with the antenna being deployed easily and conveniently.
When one considers the so-called discone antennas such as those described in U.S. Pat. Nos. 4,851,859; 6,369,766; 3,983,561; 4,623,895; and 3,987,456 it will be appreciated that, for instance at a low frequency cutoff of 30 megahertz, the cone, which is typically one quarter wavelength in height, is in the order of 8 feet tall. If one seeks to lower this low frequency cutoff from 30 megahertz to 20 megahertz as is sometimes required, the height of the discone would grow from 8 feet to 11 feet.
In the past, discone antennas for low frequency bands were made either from sheet metal cones or from multiple wire rods such as described in an article by V. Lakshminarayana, Yog Raj Kubba and Me Madhusedan, entitled “Wide Band Discone Antenna” published on p. 57 of the March–April 1971 issue of the Indian Journal of “Electro-Technology.”
From U.S. Pat. No. 3,987,456, we find the original discone antenna was described in U.S. Pat. No. 2,368,663 filed on May 15, 1943.
What will be appreciated is that whether the cone is made of sheet metal or whether it is made from rods which extend in a cone-shaped configuration from a central hub, the size necessary to provide a low frequency cutoff of 20 megahertz requires considerable real estate and considerable antenna height.
It is also important to understand that for such low frequency cutoff applications the disc diameter has to be 0.7 times the height of the cone.
What will be immediately appreciated is that such a structure is not easily portable and is not deconstructable for transportation in any easy way, making deployment of base stations in battlefield scenarios somewhat difficult. Moreover, the overall size of such antennas presents a highly visible target which is easily recognized both in the optical region of the electromagnetic spectrum and by radar.
For these reasons, what is required is a relatively small, ultrawide-band antenna which can go down to as low as 20 megahertz, in which the VSWR is less than 3:1. Moreover, the gain of such an antenna should be at least −3 dBc at the low end.
For those types of discone antennas which utilize rods extending out to complete the cone, it has been found that the height above ground is indeed a factor in tuning of the antenna. With a detuned antenna the VSWR can go from 3:1 to 15:1 by simply displaying the antenna at some point at or adjacent the earth's surface.
There is also a concern when multiple discone antennas are utilized to cover increasingly high bands, and that is the method of feeding such additional discone antennas without detuning the original low frequency band discone antenna. It will be appreciated that by merely passing coaxial cable up through the lower disc to connect to multiple antennas above the disc, the mere passage of the coax through the feed point of the disc causes detuning.
Thus, for a variety of reasons, there is a requirement for a small, field-deployable wide-band antenna immune from ground effects and small enough to be collapsible while at the same time presenting a reduced target when fully deployed.